Limited proteolysis of 3-phosphoglycerate kinase without loss of enzymic activity.

During tryptic digestion of pig muscle 3-phosphoglycerate kinase in the presence of 3-phosphoglycerate both the decrease of enzymic activity and the release of trichloroacetic acid-soluble peptides occur after a pronounced lag period. During this lag phase the native enzyme molecule is split into two fragments with molecular masses of about 30 and 18 kDa, as detected by SDS-PAGE. Under non-denaturing conditions, however, these fragments are held together by non-covalent forces and constitute an active, nicked enzyme molecule. In the absence of substrates or in the presence of MgATP the kinetics of tryptic digestion is apparently a single first order reaction leading to the formation of peptides with molecular masses of less than 10 kDa.